Embodiments of the invention relate to bipolar plates for fuel cells, and more particularly, to an improved method for depositing a thin gold coating on bipolar plates which reduces the porosity of the resulting gold coating, improves the durability of the gold coating, and reduces corrosion of the underlying bipolar plates.
In recent years, vehicle manufacturers have been increasingly pursuing fuel cell power sources due to their efficient operation and reduced emissions. A leading fuel candidate for use in transportation applications is a hydrogen/air proton exchange membrane fuel cell (PEMFC), which comprises a polymer membrane (e.g., a proton exchange membrane) that is positioned between a pair of gas diffusion media layers and catalyst layers. A cathode plate and an anode plate are positioned at the outermost sides adjacent the gas diffusion media layers. A catalytic coating is deposited on opposing sides of the membrane, forming a membrane electrode assembly (MEA). All of these components collectively form the cell unit.
Typically, several fuel cells are combined in a fuel cell stack to generate the desired electrical output. For an automotive fuel cell stack, the stack may include about two hundred or more fuel cells. In this arrangement, two adjacent cell units can share a common polar plate, which serves as the anode and the cathode for the two adjacent cell units it connects in series. Such a plate is commonly referred to as a bipolar plate, which typically includes a flow field therein to enhance delivery of the reactant gases, e.g., hydrogen and oxygen, to the associated cells.
Metallic bipolar plates are preferred for use because they are electrochemically stable, electrically conductive, and inexpensive. In addition, they can be made very thin (e.g., <0.25 mm) and can be formed into a final shape by inexpensive metal forming techniques, such as stamping. Stainless steel is commonly used to form bipolar plates. However, stainless steel is susceptible to corrosion in the humid fuel cell stack environment that includes both oxidizing and reducing conditions.
An active corrosion process in a fuel cell stack can increase the membrane resistance and the contact resistance of the bipolar plates, reducing the electrical conductivity/power density of the stack. The resulting corrosion products can also lead to chemical degradation of other fuel cell components. In order to protect the metal bipolar plates from corrosion and reduce contact resistance, the plates are often electroplated with a thin noble metal coating such as gold or a metal selected from the platinum metal group (PGM). The protective coatings are electrically conductive and have a thickness ranging from 5 to 10 nm. However, it has been found that even with the protective noble metal coatings, the underlying bipolar plates are subject to corrosion over time. Furthermore, the thin coatings suffer from significant degradation, especially on the cathode side of the fuel cell, where air enters the stack with pollutants such as iodide, bromide, chloride, thiosulfate, thiourea or mixtures thereof that have the potential of dissolving the coating, affecting the integrity of the bipolar plate over time.